The web's password forms read the server's minimum length

The server's MIN_PASSWORD_LEN was 8, and the web wrote 8 out five times: two
checks and three placeholders. The constant moves beside the other policy numbers
in settings.py (auth.py imports it), /api/config serves it as
min_password_length, and the config store hands it to Register, Reset and
Account. 8 stays only as the fallback until the config answers.

DRY pass #2, batch 3 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/AccountView.vue

@@ -264,7 +264,7 @@ onMounted(() => {
         type="password"
         label="New password"
         autocomplete="new-password"
-        placeholder="At least 8 characters"
+        :placeholder="`At least ${config.minPasswordLength} characters`"
         :error="passwordError"
         required
       />